The pulse tube refrigerator is a cryocooler, similar to Stirling and Gifford-McMahon refrigerators, that derives cooling from the compression and expansion of gas. However, unlike the Stirling and Gifford-McMahon (G-M) systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube (called a gas piston) that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in many applications, both military and commercial.
Cryogenic temperatures such as those achievable using two stage pulse tube refrigerators, are highly desirable in such commercial applications as cooling the superconducting magnets used in magnetic resonance imiaging (MRI) systems to 4 K or for cooling cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers, to 10 K.
Smaller cryocoolers are desirable in the most common applications to which pulse tube refrigerators lend themselves, such as semiconductor fabrication chambers, where continual efforts are made to reduce component size. Conventional two-stage pulse tube refrigerators, while capable of achieving two-stage refrigeration (e.g. 4 K and 10 K), require a relatively large buffer volume(s) for the two stages and are potentially less compact than Stirling or G-M refrigerators, in which the two stages require no buffer volume. Thus, any size reduction in pulse tube refrigerators is highly desirable, especially in two-stage designs that utilize one or more buffer volumes. What is needed is a way to design a more compact two-stage pulse tube refrigerator.
Conventional cryocoolers, such as Stirling and G-M refrigerators, include a moving displacer, which necessitates the inclusion of elements such as seals in the expansion space; this presents reliability problems and necessitates maintenance of such systems at regular intervals. The typical interval of 12,000 to 15,000 hours between maintenance is not a long time considering that many applications require the cryocoolers to operate indefinitely. It is desirable in such applications to strive for maintenance-free cryocooler designs. What is needed is a way to increase the maintenance interval and the reliability of a cryogenic refrigerator.
The exclusion of moving parts in the cold end of pulse tube refrigerators results in a great reduction in the level of vibration when compared to systems that are cooled by more conventional refrigerators, such as G-M and Stirling systems. The quality and uniformity of the chips produced in semiconductor fabrication vacuum chambers, in which pulse tube refrigerators may be used in cryopumps to xe2x80x9cfreeze outxe2x80x9d or purge gases, may be greatly affected by the vibration of components within the chamber, which is likely to stir up dust and other particulate matter. Likewise, pulse tube refrigerators lend themselves nicely to MRI applications, in which a large superconducting magnet must remain cooled to as low as 4 K. Even the slightest vibration of any metal component in the magnetic field produced by the superconducting magnet results in interference and degrades the quality of the produced image. What is needed is a way to minimize vibration in applications requiring two-stage cryogenic refrigeration.
Conventional pulse tubes with single or double orifice control use large buffer volumes to get good efficiency, or xe2x80x9cfour valvexe2x80x9d control to eliminate or minimize the size of the buffer volume but at the expense of efficiency. What is needed is away to design a compact pulse tube with good efficiency.
Gao et al., U.S. Pat. No. 5,974,807, entitled xe2x80x9cPulse tube refrigerator,xe2x80x9d describes a pulse tube refrigerator capable of generating cryogenic temperatures of below 10 K that includes first and second refrigeration stages. Each stage includes a pulse tube and an associated regenerator provided at the low temperature side of the pulse tube. A pressure fluctuation generator having a compressor and a first to a fourth valve is provided at the high temperature side of each regenerator. The high temperature sides of each pulse tube are connected by a continuous channel, while the high temperature sides of each pulse tube and the high temperature sides of each regenerator are connected by a by-pass channel. A magnetic material having a rare-earth element and a transition metal is used as a regenerative material for the regenerator.
When pressure fluctuation is generated in each pulse tube at the phase difference angle of 180 degrees, respectively, a working gas is transferred between the high temperature sides of each pulse tube by an active valve, thereby optimizing the phase angle between the pressure fluctuation in each pulse tube and the displacement of the working gas. The flow amount of the operating gas sent to each pulse tube from the regenerator is limited using a fixed orifice in the by-pass channel.
This patent describes active and passive inter-phase control with fixed restrictors for the second orifices. No buffer volume is included. This is possible because there two identical two-stage pulse tubes that are interconnected so the volumes and temperatures match.
Matsui et al., U.S. Pat. No. 5,845,498, entitled xe2x80x9cPulse tube refrigerator,xe2x80x9d describes a pulse tube refrigerator where the cryostat includes regenerators and pulse tubes. Each regenerator has a cold stage at an upper end thereof. Each pulse tube has a low-temperature end portion at a lower end thereof and a high-temperature end portion thereof, the low-temperature end portion being located lower than the cold stage. The cold stage and the low-temperature end portion are connected to each other through a line whose cubic volume is substantially negligible in comparison with that of the pulse tube. Since the pulse tube has working gas of relatively high density in an upper portion thereof and working gas of relatively low density in a lower portion thereof, there is no convection of working gas induced by the gravity.
This patent exemplifies the problems of applying prior art concepts to creating a configuration that is preferred for cooling cryopumps, namely having the valve mechanism below the cryopump housing. The hot end of a pulse tube has to be above the cold end in order to avoid serious convection losses in the pulse tube. This patent describes several different conventional control mechanisms for single warm regenerator designs (no inter-phase control). FIG. 2 illustrates the problems of having large dead volumes in connect tubes 36, 37, and 38, which are needed to keep the warm end of the pulse tube above the cold end with the valve mechanism below the pulse tube. The conventional construction shown as prior art in FIG. 1 is suitable for cooling a cryopump if there is room for the valve mechanism above the cryopump housing.
Matsui et al., U.S. Pat. No. 5,711,156, entitled xe2x80x9cMultistage type pulse tube refrigerator,xe2x80x9d describes a multistage G-M type pulse tube refrigerator comprising a regenerator-side pressure oscillation generator, first regenerator connected to the regenerator-side pressure oscillation generator, first cold head connected to the low temperature side of the first regenerator, a first pulse tube having one end connected to the first cold head and the other end connected by way of a first flow regulating mechanism to a first pulse tube-side phase shifter, second regenerator having one end connected to the first cold head and the other end connected to the second cold head, a second pulse having one end connected to the second cold head and the other end connected to second pulse tube-side phase shifter by way of second flow regulating mechanism, in which the first pulse tube-side phase shifter and the second pulse tube-side phase shifter are controlled independently of each other. The pulse tube refrigerator operates while setting the phase angle of the pulse tube-side phase shifter to xe2x88x9250 degrees to a xe2x88x92120 degree phase angle relative to the regenerator-side pressure oscillation generator, while setting the phase angle of the second pulse tube-side phase shifter 15 degrees to a xe2x88x9290 degree phase angle.
This patent describes a two-stage pulse tube with a single warm regenerator, (no inter-phase control). It uses the xe2x80x9cfour valvexe2x80x9d method to control the flow of gas to each stage without having any buffer volumes. The valve timing may be different for each stage. This patent shows examples of conventional multi-ported rotary valves, FIGS. 4, 5, and 6.
Ohtani et al., U.S. Pat. No. 5,335,505, entitled xe2x80x9cPulse tube refrigerator,xe2x80x9d describes a pulse tube refrigerator, comprising a regenerator having an inlet port and an outlet port, a pulse tube having one end portion connected in series to the outlet port of the regenerator, a gas compressor connected to the inlet port of the regenerator, a first valve disposed between the discharge port of the gas compressor and the inlet port of the regenerator, a second valve disposed between the suction port of the gas compressor and the inlet port of the regenerator, a first valve controller for selectively opening/closing alternately the first and second valves to permit a high pressure coolant gas discharged from the discharge port of the gas compressor to be guided into the pulse tube through the regenerator and, then, to permit said coolant gas to be sucked into the gas compressor through the suction port thereof via the reverse passageway so as to generate coldness, a third valve disposed between the other end portion of the pulse tube and the discharge port of the gas compressor, a fourth valve disposed between the other end portion of the pulse tube and the suction port of the gas compressor, and a second valve controller serving to open/close the third and fourth valves in relation to the opening/closing of the first and second valves.
This patent covers the xe2x80x9cfour valvexe2x80x9d control concept, with and without a buffer volume. It describes single warm regenerator designs, (no inter-phase control). Inline designs with the valve mechanism below the pulse tubes and the hot end of the pulse tubes up are shown.
Zhu, S. and Wu, P., xe2x80x9cDouble inlet pulse tube refrigerators: an important improvementxe2x80x9d, Cryogenics, vol. 30 (1990), p. 514 describe a second orifice and how it improves the performance of a single stage pulse tube.
A. Watanabe, G. W. Swift, and J. G. Brisson, Superfluid orifice pulse tube below 1 Kelvin, Advances in Cryogenic Engineering, Vol. 41B, pp. 1519-1526 (1996) describe inter-phase control of a very low temperature Stirling cycle cooler that has one passive orifice between two identical pulse tubes.
J. L. Gao and Y. Matsubara, An inter-phasing pulse tube refrigerator for high refrigeration efficiency, in: xe2x80x9cProceedings of the 16th International Cryogenic Engineering Conferencexe2x80x9d, T. Haruyama, T. Mitsui and K. Yamafriji, ed., Eisevier Science, Oxford (1997), pp. 295-298, describe identical dual 1, 2, and 3 stage pulse tubes with single active interconnect valves.
C. K. Chan, and E. Tward, Multistage pulse tube cooler, U.S. Pat. No. 5,107,683, Apr. 28, 1992.
This patent describes a second stage pulse tube that extends from the coldest temperature to ambient temperature with no intermediate regenerator material.
C. K. Chan, C. B. Jaco, J. Raab, E. Tward, and M. Waterman, Miniature pulse tube cooler, Proc.7th Int""l Cryocooler Conf., Air Force Report PL-CPxe2x80x9493-1001 (1993) pp. 113-124, describe a Stirling single stage pulse tube that is inline, so the hot end of the pulse tube is remote from the regenerator inlet. It has double orifice control. Heat from the hot end of the pulse tube and buffer are rejected to the base at the regenerator inlet by conduction through the buffer housing which extends the full length of the pulse tube. The hot end of the pulse tube is not attached to the vacuum housing so the entire pulse tube assembly can be easily removed.
Y. Matsubara, J. L. Gao, K. Tanida, Y. Hiresaki, and M. Kaneko, An experimental and analytical investigation of 4 K pulse tube refrigerator, Proc.7th Int""l Cryocooler Conf., Air Force Report PL-CPxe2x80x9493-1001 (1993) pp. 166-186, describe the xe2x80x9c4 valvexe2x80x9d control concept and describes why it increases the PV work produced in the cold end of the pulse tube relative to double orifice control.
It is an object of the present invention to provide a more compact two-stage pulse tube refrigerator by minimizing the size of the buffer volume.
It is an object of the present invention to provide a way to design a more efficient compact two-stage pulse tube refrigerator by using inter-phase control in combination with a buffer volume.
It is an object of the present invention to minimize vibration in a cryogenic refrigerator.
It is an object of the present invention to provide increased reliability of a cryogenic refrigerator.
It is an object of the present invention to provide a buffer tank to compensate for flow differences between the pulse tubes of the first and second stages.
It is an object of the present invention to reduce the number of regenerators is from four to two and the number of pulse tubes from four to two.
It is an object of the present invention to use four-valve control in combination with inter-phase control so the valve timing is the same for each stage.
The present invention addresses how a pulse tube refrigerator can be effectively and efficiently incorporated in a cryopump. The present invention addresses issues of compactness of the expander, low vibration, high reliability, and a preference for the valve mechanism to be on the bottom or side of the cryopump.
Refrigerators of the present invention can be adapted to cooling cryopump panels at two different temperatures in a way that is more compact and efficient than prior art pulse tubes. One very important attribute is the option of adding a buffer tank with minimal volume to the inter-phase connection to compensate for flow differences between the two stages of the pulse tube.
A first difference between the present invention and the prior art is the present invention""s ability to design the first and second stages to use differing amounts of gas, whereas the prior art have no design flexibility.
A second difference between the present invention and the prior art is that certain embodiments of the present invention includes a buffer volume that is shared between the pulse tubes of the first and second stages, whereas the prior art describes a double orifice control including a much larger buffer volume.
In certain embodiments of the present invention the flow in the two stages is balanced and the required buffer volume is 0.